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Unformatted text preview: 18.03 Problem Set 4: Solutions 13. (a) The system is underdamped; the roots are not real. (b) Since the system is underdamped, the solutions are of the form x = Ae- bt/ 2 cos( d t- ). Then x = Ae- bt/ 2 ((- b/ 2) cos( d t- ) + d sin( d t- )). The term following the exponential is sinusoidal, with circular frequency d . Therefore it vanishes every / d seconds, and this is the time between a maximum and a minimum, or half of a cycle. We have discovered that / d = 4 / 2 = 2, or d = 2 radians/sec. Since the roots of the characteristic polynomial are- b 2 d i , we have discovered that their imaginary parts are / 2. (c) For any t , A cos( d t- ) = A cos( d ( t +4)- ), since d = 2 . So the ratio x ( t +4) /x ( t ) is given by the ratio of the other terms: x ( t + 4) /x ( t ) = e- b ( t +4) / 2 /e- bt/ 2 = e- 2 b . This tells us that 1 / 2 = e- 2 b , or b = (ln 2) / 2: The real part of the roots is- (ln 2) / 4....
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- Spring '09